Methamphetamine self-administration is associated with persistent biochemical alterations in striatal and cortical dopaminergic terminals in the rat

Abstract

Methamphetamine (meth) is an illicit psychostimulant that is abused throughout the world. Repeated passive injections of the drug given in a single day or over a few days cause significant and long-term depletion of dopamine and serotonin in the mammalian brain. Because meth self-administration may better mimic some aspects of human drug-taking behaviors, we examined to what extent this pattern of drug treatment might also result in damage to monoaminergic systems in the brain. Rats were allowed to intravenously self-administer meth (yoked control rats received vehicle) 15 hours per day for 8 days before being euthanized at either 24 hours or at 7 and 14 days after cessation of drug taking. Meth self-administration by the rats was associated with a progressive escalation of daily drug intake to 14 mg/kg per day. Animals that self-administered meth exhibited dose-dependent decreases in striatal dopamine levels during the period of observation. In addition, there were significant reductions in the levels of striatal dopamine transporter and tyrosine hydroxylase proteins. There were also significant decreases in the levels of dopamine, dopamine transporter, and tyrosine hydroxylase in the cortex. In contrast, meth self-administration caused only transient decreases in norepinephrine and serotonin levels in the two brain regions, with these values returning to normal at seven days after cessation of drug taking. Importantly, meth self-administration was associated with significant dose-dependent increases in glial fibrillary acidic protein in both striatum and cortex, with these changes being of greater magnitude in the striatum. These results suggest that meth self-administration by rats is associated with long-term biochemical changes that are reminiscent of those observed in post-mortem brain tissues of chronic meth abusers.

Figure 1. Changes in METH intake and body weight over the course of self-administration in rats.

(A) Escalation of METH intake in rats. Symbols indicate average daily amount of actively self-administered METH during each of 8 consecutive daily sessions (means ± SEM; n = 11). Rats increased METH intake on days 3–8 compared with days 1 and 2, and days 7–8 showed further increases in intake compared with day 3. Data analyzed by one-way ANOVA for repeated measures, followed by Tukey's multiple comparison test: *** p<0.001 in comparison with sessions 1 and 2, # p<0.05 in comparison with session 3. (B) METH caused significant decreases in body weight in the animals that actively self-administered the drug (means ± SEM; n = 9). Data analyzed by two-way ANOVA for repeated measures, followed by Tukey's multiple comparison test: * p<0.05; *** p<0.001 in comparison to saline group.

Figure 2. Effects of METH self-administration on monoamine levels in the striatum.

METH caused persistent decreases in striatal DA levels (A), which negatively correlated with total drug intake when data at 24 hours, 7 and 14 days after cessation of self-administration were combined (B) and at 14 days post-drug (C). However, only transient changes were found in the levels of NE and 5-HT (D). Data shown as mean ± SEM. *, p<0.05; **, p<0.01; ***, p<0.001 vs control. Data were analyzed by ANOVA followed by PLSD, n = 7−11 per group. Correlation analysis was done by regression analysis.

Figure 3. Effects of METH self-administration on monoamine levels in the cortex.

METH self-administration resulted in long-term reductions in DA concentrations in the cortex (A). DA levels showed negative correlations with total METH intake at the 24 hour, 7 and 14 days time-points (B) and at 14 days after cessation of self-administration (C). METH caused only transient reductions in NE and 5-HT concentrations in the cortex (D). Data shown as mean ± SEM. *, p<0.05; **, p<0.01 vs control. Data were analyzed by ANOVA followed by PLSD, n = 7−11 per group. Correlation analysis was done by regression analysis.

Figure 4. METH self-administration causes reductions in DAT and TH protein levels in the striatum.

(A) Representative immunoblots show expression of TH, DAT and 5-HTT protein levels in the striatum 14 days after last METH self-administration session. (B) Quantitative analyses of immunoblots reveal significant decreases in TH and DAT, but no change in 5-HTT expression in the METH-treated rats. Data presented as mean ± SEM. ** p<0.01 vs control. Data were analyzed by ANOVA followed by PLSD, n = 7−11 per group.

Figure 5. METH self-administration resulted in decreased expression of DAT and TH proteins in the cortex.

(A) Immunoblots showing TH, DAT and 5-HTT protein expression in the cortex 14 days after cessation of METH intake. (B) Quantification of METH toxic effects demonstrates significant decreases in TH and DAT, but not in 5-HTT protein levels. Data shown as mean ± SEM. * p<0.05; ** p<0.01 vs control. Data were analyzed by ANOVA followed by PLSD, n = 7−11 per group.

Figure 6. Effects of METH self-administration on GFAP expression in the striatum and cortex.

Representative Western blot analyses show that the METH self-administering rats had increased GFAP levels in their striata (A) and cortices (D). Quantitative analyses of the Western blots show significant increases in GFAP levels in the METH-treated rats (B, E). Key to statistics: ** p<0.01, *** p<0.001 (ANOVA, n = 7 per group). GFAP expression showed significant positive correlation with METH intake in the striatum (C) and cortex (F).